The present invention relates to computers and, more particularly, to schemes for reducing RF emissions from computers. A major objective of the present invention is to provide for reduced RF emissions from computers.
Much of modern progress is associated with advances in computer technology that have led to the increasing prevalence of both general-purpose computers and special-purpose microprocessor-based products. A problem with such devices is that they can emit radio-frequency (RF) electromagnetic energy that can interfere with the operation of other devices that are located nearby.
Several approaches have been applied to reduce the emissions from a computer. One general design approach is to minimize the use of structures that are likely to emit RF energy. For example, structures that inadvertently act like antennas, transmission lines, and other transmission systems should be avoided. Transmission systems typically include two conductors spaced by dielectric material, typically air in a computer. The transmission system is typically elongated or otherwise dimensioned so that electrical energy can be propagated along it. Such structures, especially under resonant conditions, are likely sources of RF emissions. Avoiding unintended transmission systems thus can limit RF emissions. This structural approach is of course limited, since some required structures, like cables, have inherent transmission system characteristics.
Since some RF emissions are inevitable, shielding is generally arranged to prevent their escape from the computer. Surrounding RF-emitting elements with grounded metal shielding can be an effective approach to preventing RF emissions from escaping. Many computers are housed in metal cabinets. In addition, cables, both within a computer and extending out of a computer, can have metal shielding surrounding signal-bearing conductors. Long metal runs within PCB boards can be shielded by placing them between metal layers. Currents induced in the shielding remain associated with their producing signal-bearing conductors resulting in a minimization of fields that can escape the shield. The resonances that may be set up within the shielded environment and which could yield the strongest emissions are thus isolated from the outside world.
While shielding can be effective in principle, emissions can escape through breaches in the shielding. In general, RF energy can readily escape through breaches dimensionally comparable with the RF wavelengths. RF wavelenghts are inversely proportionally to RF frequencies and, thus, are becoming shorter as base clock speeds are increased. Some breaches are often required by design: e.g., slots for inserting devices and media, ventilation holes, and metal-to-metal contacts. Some breaches result from damage to shielding. For example, flexing cables during installation can breach cable shielding either at cable ends or along the cable length. In either case, the breaches can provide escape routes for RF energy.
The shortest wavelength RF emissions are often associated with harmonics over base frequencies. The harmonics are associated with frequencies that are multiples of the base frequencies, and have correspondingly shorter wavelengths. Filtering and other wave-shaping techniques can smooth sharp transitions to reduce the energy associated with high-frequency harmonics so that much of the RF energy is at longer wavelengths that are effectively blocked by the computer shielding.
However, to meet an incessant demand for higher performance, switching frequencies are increasing, e.g., well beyond 100 MHz, so that even RF energy associated with base frequencies can escape through conventionally dimensioned slots, holes, and vents. Thus, the basic transmission avoidance, shielding, and filtering approaches must be supplemented.
In the case of cables, ferrite material can be applied to external cables and to internal cables where they interface with a PCB board. Current-carrying cables generate magnetic fields. The ferrite material operates on these magnetic fields, attenuating the energy by magnetic field loss. The ferrites, however, are bulky and inflexible, and thus inconvenient where space is at a premiumxe2x80x94as is often the case near the signal source. Furthermore, optimum performance may require precise positioning, which can be difficult to guarantee, especially where the computer interior is user accessible. Even with precise positioning, certain frequencies may not be attenuated. For example, the effectiveness of ferrite modules generally drops off above 500 MHz.
In summary, the structural-design, shielding, filtering, and magnetic-field attenuating approaches can be used in combination to greatly reduce RF emissions from computer systems. However, the collective effectiveness of these approaches can still be inadequate, especially, with unwanted RF emissions approaching and exceeding one gigahertz. What is needed is another approach to reducing RF emissions, especially, at frequencies from 100 MHz to 1 GHz and beyond.
The present invention, in a sense, opposes the prior-art approach of reducing RF emissions by avoiding transmission systems. The present invention encourages the use of transmission systems for use in reducing RF energy. However, where the prior art used non-lossy transmission systems by default, the present invention calls for electrically lossy transmission systems to dissipate RF energy.
An electrically lossy transmission system can include two conductors separated by electrically lossy dielectric material. Herein, xe2x80x9celectrically lossyxe2x80x9d requires an electrical loss tangent (dissipative factor) of at least 0.1 for 100 MHz waveforms. Preferably, the electrical loss tangent is greater than the magnetic loss tangent for the material. At least one of the conductors serves as a return path for RF current in the other conductor. The invention provides for including electrically lossy dielectric material between existing conductors that might act as transmission lines, and for disposing metal-backed electrically lossy dielectric adjacent to conductors to define an incorporating electrically lossy transmission system.
The transmission system should extend at least four times the conductor spacing, which should be no more than about 1 centimeters (cm) apart. The space between the conductors should be substantially (at least 90% of the separation) filled with dielectric material. Preferably, the dielectric material is in contact with both conductors. While the space between the conductors is preferably filled entirely with electrically lossy dielectric, some thickness of dielectric can be non lossy. For example, a composite dielectric structure can result when electrically lossy dielectric is disposed on a conductor which has a non-lossy dielectric protective coating in place. When not all of the dielectric is lossy, it is preferable that the lossy dielectric occupy a relatively large portion of the distance between the conductors and have a relatively low dielectric constant (compared to the non-lossy dielectric) so that most of the voltage drop between the conductors occurs within the lossy dielectric.
The electrically lossy dielectric can be a flexible dielectric material (plastic, rubber, or foam) for good conformance to conductive surfaces to which it may be applied. The electrically lossy character of the dielectric can be achieved by a colloidal resistive graphite particles dispersed throughout the dielectric. The dispersion can be uniform or non-uniform. In the later case, the density of the graphite particles can vary transversely between the conductors or longitudinally along the conductors. In the latter case, the corresponding changes in impedance along a transmission path can result in multiple reflections, each of which results in additional dissipation of unwanted RF energy. Such enhanced dissipation can also be achieved by varying the dielectric thickness or the dielectric constant longitudinally.
This invention provides for dissipating unwanted RF energy by operating on its electric field. In general, propagating RF energy establishes maximum voltage standing waves on all metallic elements when these elements have dimensions of one-quarter wavelength or multiples thereof and with ends that are not terminated in the characteristic impedance of the propagating energy. It is these critical dimensions that offer the optimum potential for standing-wave generation between the transmission elements. These standing waves provide the opportunity of dissipating all the propagating energy.
Preferably, at least one metal element is closely coupled to the source of the unwanted energy. At least one other metal element provides a local return path for the RF originating energy. For optimum results, the electrically lossy material should have bulk resistivity and a dielectric constant optimized to provide the largest voltage drop across it when located intimately between the metal elements acting as the transmission system. The material need not actually contact a metal element if there is an adequate dielectric material over the metal that provides for minimizing the voltage drop across it compared to that across the electrically lossy structure.
The electrically lossy transmission systems dissipate RF energy, so there is less RF energy available to be emitted. The invention shares this advantage with the dissipation of magnetic fields approach. The latter has the advantage of not relying on adjacent metal structures or adding metal to achieve RF dissipation.
On the other hand, the present invention provides for materials that are more economical, more flexible, and more readily adapted to the computer environment. Electrically lossy materials can be based on graphite, which is much less expensive than the ferrite used for magnetically lossy materials. The electrically lossy material can be readily fabricated in sheets, so that they can be readily applied to planar surfaces within a computer. Also, available electrically lossy dielectrics are compressible, so that can be press fitted to target conductors to ensure good contact. In the case of flat surfaces, it can be difficult to apply an effective magnetic loss medium (which should surround any currents), but it is relatively easy to apply a flat sheet of electrically lossy material.
While the electrically lossy dielectric and the metal-plus-lossy-dielectric can be applied on an ad hoc basis, the invention provides for more optimal RF emission reductions where a computer is designed from the ground up to include electrically lossy transmission systems. In any event, the invention can be used effectively with other RF reduction methods including the use of magnetic lossy materials, structural shielding, and waveshaping. These and other features and advantages of the invention are apparent from the description below with reference to the following drawings.